Non-uniform two dimensional electron gas profile in III-Nitride HEMT devices

ABSTRACT

A HEMT device has a substrate; a buffer layer disposed above the substrate; a carrier supplying layer disposed above the buffer layer; a gate element penetrating the carrier supplying layer; and a drain element disposed on the carrier supplying layer. The carrier supplying layer has a non-uniform thickness between the gate element and the drain element, the carrier supplying layer having a relatively greater thickness adjacent the drain element and a relatively thinner thickness adjacent the gate element. A non-uniform two-dimensional electron gas conduction channel is formed in the carrier supplying layer, the two-dimensional electron gas conduction channel having a non-uniform profile between the gate and drain elements.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.13/478,402 filed on May 23, 2012 and entitled “A Non-Uniform LateralProfile Of Two-Dimensional Electron Gas Charge Density In Type IiiNitride Hemt Devices Using Ion Implantation Through Gray Scale MasK andto U.S. patent application Ser. No. 13/478,609 filed on May 23, 2012 andentitled “HEMT GaN Device with on Uniform Lateral Two-DimensionalElectron Gas Profile and Process for Manufacturing the Same”.

TECHNICAL FIELD

This invention relates to III-Nitride HEMT devices having a non-uniformTwo-Dimensional Electron Gas (2DEG) profile in the carrier supplyinglayer thereof.

BACKGROUND

III-Nitride High Electron Mobility Transistor (HEMT) devices are oftenused in power applications and/or high temperature applications in RFcircuits and in other applications, including in power supplies forelectrically powered motor vehicles.

A design trade-off between the on-state resistance (R_(on)) andbreakdown voltage (BV) of a HEMT can be improved significantly byfollowing the teachings contained herein. Since the relation between theBV and R_(on) is at least quadratic, improvement in the BV for a givendrift

region length results in a significant improvement in the figure ofmerit (FOM) of the device, defined as BV²/R_(on).

HEMTs utilize two semiconductor materials with different band-gaps,forming an electron potential well at a heterointerface between the twosemiconductor materials, which materials might be, for example, AlGaNand GaN. The potential well confines electrons and defines atwo-dimensional electron gas (2DEG) conduction channel. Due to thetwo-dimensional nature of the electrons in the conduction channel, thecarrier mobility is enhanced.

Prior art III-Nitride HEMTs utilize a uniform 2DEG density which resultsin a peak electric field under or near the gate region. The electricfield distribution tends to be closer to a triangular shape than to amore desirable trapezoidal shape which reduces the BV per unit driftregion length of the device. The use of a field plate and/or multi stepfield plates are some of the techniques that are used in the prior artto improve the electric field distribution but these techniquestypically result in multiple peaks and suffer from less than ideal flatfield distribution (they can exhibit a saw tooth type profile) whichalso adds to the gate to drain capacitance. In addition, processcomplexity and cost typically increase with the number of field platesteps (levels) utilized.

The prior art includes:

Furukawa, U.S. Pat. No. 7,038,253 issued on May 2, 2006 discloses a GaNbased device that represents state of the art GaN on Si technology whichuses a uniform 2DEG profile in the drift region. In the absence of anyfield shaping technique it is expected that the breakdown and dynamicRdson performance of the device of this patent will be limited by alocalized increase in the electric field under the gate region thusrequiring over design of the device which degrades the Figure of Merit(FOM) that can be achieved by such a structure.

H. Xing et al. have proposed a device structure that was published in apaper entitled “High Breakdown Voltage AlGaN/GaN HEMTs achieved byMultiple Field Plates”, (see H. Xing, Y. Dora, A. Chini, S. Hikman, S.Keller and U. K. Mishra, “High Breakdown Voltage AlGaN—GaN HEMTsAchieved by Multiple Field Plates,” IEEE Electron Device Letters, IEEEELECTRON DEVICE LETTERS, VOL. 25, NO. 4, pp. 161-163, April 2004), whichutilizes a field shaping technique that used multiple field plates toimprove the electric field distribution, however, this technique is lessfavorable than the technology disclosed herein since multiple fieldplates will not achieve a uniform electric field (will have a saw toothtype distribution) and will increase the gate to drain capacitance. Inimplementing such structure increases device complexity and cost.

C. M. Waits, R. Ghodssi, and M. Dubey, “Gray-Scale Lithography for MEMSApplications”, University of Maryland, Department of Electrical andComputer Engineering, Institute for Advanced Computer Studies, CollegePark, Md., USA, 2006.

W. Henke, W. Hoppe, H. J. Quenzer, P. Staudt-Fischbach and B. Wagner,“Simulation and experimental study of gray-tone lithography for thefabrication of arbitrarily shaped surfaces,” Proc. IEEE Micro ElectroMechanical Syst. MEMS 1994, Oiso, Japan, pp. 205-210.

C. M. Waits, R. Ghodssi, M. H. Ervin, M. Dubey, “MEMS-based Gray-ScaleLithography,” International Semiconductor Device Research Symposium(ISDRS), Dec. 5-7, 2001, Washington D.C.

In another aspect the present invention relates to a HEMT devicecomprising: a substrate; a buffer layer disposed above said substrate; acarrier supplying layer disposed above said buffer layer; a gate elementpenetrating said carrier supplying layer; a drain element disposed onsaid carrier supplying layer; wherein the carrier supplying layer has anon-uniform thickness between said gate element and said drain element,the carrier supplying layer having a relatively greater thicknessadjacent the drain element and a relatively thinner thickness adjacentthe gate element.

In yet another aspect the present invention relates to a HEMT devicehaving a non-uniform two-dimensional electron gas conduction channelformed in a carrier supplying layer of the HEMT device between a gateelement of the HEMT device and a drain element of the HEMT device, theHMET device also having a constant electric field distribution betweensaid gate element and said drain element.

In still yet another aspect the present invention relates to a HEMTdevice comprising: a substrate; a buffer layer disposed above saidsubstrate; a carrier supplying layer disposed above said buffer layer; agate; a drain disposed on said carrier supplying layer; wherein a twodimensional electron gas (2DEG) is formed between the gate and the drainand wherein the carrier supply layer is configured to adapt the 2DEGsuch that a variation in electric field strength as a function of adistance between the gate and the drain is substantially constant.

BRIEF DESCRIPTION OF THE INVENTION

The invention is concerned with a device structure and a method ofimplementing a non-uniform two dimensional electron gas profile betweenthe gate and drain electrodes. By implementing a tapered AlGaN layer(charge supplying layer) from the gate to the drain, one can obtain amonotonically increasing 2DEG profile that results in a uniform electricfield distribution hence maximizing the FOM of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic presentation of charge density before taperingthe carrier supplying layer (preferably AlGaN), hence the uniform 2DEGdensity (depth is not drawn to scale) of a partially formed HMET device;

FIG. 1 b is a schematic presentation of charge density after taperingthe carrier supplying layer (preferably AlGaN) hence the non-uniform2DEG density (depth is not drawn to scale) of the partially formed HMETdevice of FIG. 1 a after the AlGaN layer has been tapered;

FIG. 1 c is similar to FIG. 1 b, but depicts the partially formed HMETdevice of FIG. 1 b in a perspective view as opposed to an elevationalview thereof;

FIG. 2 is a schematic presentation of charge density (depth is not drawnto scale) of the preferred embodiment of a completed HMET device of FIG.1 a after the carrier supplying layer (preferably AlGaN) has beentapered in the region between the gate and the drain;

FIG. 3 depicts are alternative method of creating a taper in the carriersupplying layer (preferably AlGaN);

FIG. 4 depicts both the electric field distribution (which is flat) andthe tapered 2DEG density for the device of FIG. 2; and

FIG. 5 depicts both the electric field distribution (with a triangularprofile) and a flat 2DEG density for prior art HEMT devices having zero,one and two field plates.

DETAILED DESCRIPTION

As depicted by FIGS. 1 a, 1 b, 1 c and 2, and in order to improve theelectric field distribution along the drift region 26 (the regionbetween the gate 30 and the drain 34) and hence improve the breakdownvoltage capability per unit drift region length of a HMET device 8, anon-uniform lateral 2DEG profile 12 along the drift region in layer 18is provided, in one preferred embodiment, by monotonically tapering theprofile (thickness or z-direction) of a carrier supplying layer 16 alonethe x-direction between the gate 30 and drain 34. The taper isidentified by numeral 14 in FIGS. 1 b and 1 c. The carrier supplyinglayer 16 may be, for example, AlGaN (but not limited to) in an AlGaN/GaNHEMT. Monotonically tapering the profile (thickness) of the carriersupplying layer 16 effectively creates a non-uniform profile of twodimensional electron gas (2DEG) 12 where the 2 DEG density increaseswith the increasing thickness of carrier supplying layer 16.

FIG. 1 a shows HMET device 8 in the process of being fabricated. Thegate, source and drain electrodes of the device 8 have not yet beenformed. The device 8 in the process of being fabricated is comprised inthis figure of a stack of III-V layers, preferably grown on a substrate10. Substrate 10 may be any of the suitable substrates that are commonlyused to grow III-Nitride materials, for example Si, Sapphire, SiC, bulksingle crystal GaN, and others. As can be seen in FIG. 1 a, substrate 10provides a supporting surface for a layer 18 of GaN material in oneembodiment (but other materials such as AlGaN may be used instead forlayer 18) or a superlattice formed of alternating layers (e.g.alternating AlGaN/GaN or alternating AlN/GaN) may be used instead forlayer 18 or a combination of C-doped GaN buffer and AlGaN back barriermay be used instead for layer 18 or any combination of theaforementioned in still other embodiments may prove to be suitable forlayer 18, which layer functions as a buffer layer in HMET device 8.

A carrier supplying layer 16 is preferably formed of AlGaN material andpreferably with a suitable Al mole fraction that typically rangesbetween 20 to 30%, and is grown or otherwise formed on buffer layer 18.FIG. 1 a also shows a layer of photoresist 22 disposed on the carriersupplying layer 16, which layer 22 has been photolithographicallyprocessed, preferably by gray scale lithography, to allow a triangularlyshaped wedge (when viewed in cross section) portion 24 to be etched awayfrom the photoresist layer 22. The layer of photoresist 22 and itstriangularly shaped wedge portion 24 is then removed during a subsequentRIE etch process which transfers the wedge pattern 24 from thephotoresist 22 and into the carrier supplying layer 16 to thereby definetaper 14 therein (see FIGS. 1 b and/or 1 c). The process is preferablyoptimized so that photoresist 22 remains in the area where the carriersupplying layer 16 layer is preferably kept intact outside the taperarea 14 so that its thickness is preferably not decreased outside of thetaper or wedge region 14 by the aforementioned RIE etch. Any remainingphotoresist 22 may thereafter be removed with a suitable chemicaletchant.

The thicker the carrier supplying layer 16 in a given position in thex-direction along the drift region 26 (see FIG. 2) where x is thehorizontal direction from the edge of gate 30 (facing the drain, wherex=0) towards the drain 34 (where x=LD at the edge of drain 34 facing thegate 30). The taper 14 in carrier supplying layer 16 (see FIGS. 1 band/or 1 c) is preferably produced by gray scale photolithography of thephotoresist layer 22 (to remove the photoresist wedge portion 24therefrom) as mentioned above. This process is followed by theaforementioned controlled RIE where initially the remaining photoresist22 is removed in the area of the wedge pattern 24 and eventually iscompletely removed either as the RIE process progresses or by thechemical etch mentioned above. The carrier supplying layer 16 under thethinner part of the wedge pattern 24 experiences a longer RIE etch timethan carrier supplying layer 16 under a thicker part of the wedgepattern 24, resulting in a profile transfer from the photoresist wedgepattern 24 to the carrier supplying layer 16. The carrier supplyinglayer's thickness is preferably uniform in a lateral direction along thelateral extents of the gate or drain regions (along the y-direction ofFIG. 1 c).

An alternative method for forming such a tapered pattern or wedge 14 inthe carrier supplying layer 16 is shown in FIG. 3. This alternativemethod involves opening windows 24′ in the photoresist layer 22 withvarying sizes where the size of the opening is a function of the lateraldistance from where the gate will be formed to where the drain will beformed (the pattern is similar to that typically used in Gray scalelithography). Since the photoresist 22 is completely removed in the openwindows 24′, the loading effect of the RIE etch will result in a fasteretch rate in larger photoresist window openings than in smaller windowopenings hence implementing a taper 14 in the carrier supplying layer 16as depicted by FIGS. 1 b and 1 c. The mask pattern in this alternativemethod is similar to a conventional gray scale mask, but rather thanrelying on the intensity of light for making different openings tocreate a tapered profile 24 in the photoresist 22, this alternativemethod relies on the loading effect of the RIE etch process to etch moreof the carrier supplying layer 16 in the wider open windows.

Irrespective of which method is used to form the taper 14, the taper 14ends at a step 20 (see FIG. 1 b or 1 c) back to the normal height oflayer 16 near where the gate 30 will be formed. The taper 14 smoothlyends where the carrier supplying layer 16 returns to its otherwisenormal height at the other end 38 of the wedge or taper 14 near (andpreferably immediately adjacent) where the drain 34 will be formed. Thegate 30, drain 34 and a passivation layer 28 with eventually occupy thisregion as shown in FIG. 2. FIG. 1 b illustrates that the taper 14 variesin the x-direction. FIG. 1 c illustrates that the taper 14 preferablydoes not vary in the y-direction.

After the tapering of the AlGaN layer 16 is completed preferably usingthe techniques discussed above with respect to either FIGS. 1 a and 1 bor FIG. 3, ohmic contacts 36 and 34 to the source and drain 2DEG regionsare formed preferably using a stack of metal lift off followed by anRapid Thermal Anneal (RTA) treatment. Thereafter a dielectric 28 isdeposited at the exposed surfaces for passivation and it is subsequentlypatterned in the source and drain contact 36 and 34 areas to open theohmic contacts 36 and 34 followed by the formation of a gate 30 stack.First a gate foot is preferably etched in the passivation dielectric 28using either a dry etching or wet etching or a combination of dry/wetetching. The techniques disclosed herein are suitable for use witheither an enhancement mode HMET device or a depletion mode HMET device.In a preferred embodiment, as in an enhancement mode device, a fluorinetreatment or a combination of fluorine treatment and gate recess (with afurther dry etch) can be performed to deplete the channel under the gate30 of its 2DEG. Thereafter a suitable gate dielectric 32 is deposited.In a preferred embodiment the gate dielectric 32 could by of Al₂O₃ oxidedeposited by atomic layer deposition (ALD), however, gate dielectricmaterials other than Al₂O₃ may be utilized and may be deposited usingmethods other than ALD including but not limited to PECVD, LPCVD,in-situ grown in MOCVD reactor, etc.

A gate 30 metal stack is then deposited and patterned. Further steps toimplement multi-step field plates can then be used where the cumulativeeffects of field shaping techniques using both non-uniform 2DEG densityprofile and multi-step field plate techniques can be combined.Additional inter-metallic dielectric and metal layers may be used toreduce the interconnect resistance particularly if the resulting HMET isa large power device.

Since the density of charge in the 2DEG region 12 is determined locallyby the thickness of the carrier supplying layer 16 at any givenposition, a non uniform 2DEG distribution is achieved by controlling theheight (thickness) of the carrier supplying layer 16 which shouldincrease as a function of distance laterally away from the gate 30 alongthe drift region toward the drain 34. See FIG. 2 where the right handside of the gate 30 structure is preferably positioned where the step 20formerly occurred and the left hand side of the drain 34 is preferablypositioned at the other end 38 of the wedge or taper 14. The dependenceof the 2DEG density on the thickness of the carrier supplying layer 16is illustrated in the following paper: Smorchkova, I. P. et al.,“Polarization-induced charge and electron mobility in AlGaN/GaNheterostructures grown by plasma-assisted molecular epitaxy,” Journal ofApplied Physics, Volume 86, Issue 8, pp. 4520-4526, October 1999, and inparticular FIG. 5 c.

From the band energy diagram of FIG. 5, which is from the paper bySmorchkova, I. P. et al identified above, it can be seen that when athin AlGaN layer is utilized as the carrier supplying layer 16, theFermi level is above the donor surface states level which results in thedonors atoms being filled and not supplying the 2DEG electrons to thewell at the GaN/AlGaN interface. As the AlGaN thickness increases theFermi level moves downwards and eventually overlaps with the donor stateenergy level resulting in some of the donors are being empty with theiroriginally compensating electrons being transferred to the 2DEG well.The further the overlap is the higher the 2DEG density is.

A flat electric field distribution is achieved using the non-uniform2DEG density concept as shown in FIG. 5 of the paper by Smorchkova, I.P. et al identified above which maximizes the lateral breakdown voltageper unit length of the drift region and improves immunity to dynamicR_(on) degradation. In contrast, as shown in FIGS. 5 a-5 c for variousconfigurations, if a uniform 2DEG density profile is used, then theelectric field profile is either triangular (no field plate) or hasmultiple peaks as shown in FIGS. 5 b and 5 c (multiple field plates).The uniform 2DEG and zero or more field plates results in a reducedbreakdown voltage BV and less immunity to dynamic R_(on) degradationcompared tapered 2DEG distribution shown in FIG. 4.

If one adds one or more field plates to the embodiment of FIG. 2 asmentioned above, that will increase the complexity of the process sinceadditional process steps are then required which will increase the costof making the device. However, adding one or more field plates to theembodiment of FIG. 2 may yield additional performance benefits. Whetherthe improvement in performance by adding one or more field plates to theembodiment of FIG. 2 is justified by the increase in cost of manufactureis a matter of design choice.

Not shown in the drawings, but as is well known, a spacer layer of AlN,for example, may be inserted between layers 16 and 18 to improve deviceelectrical performance.

This concludes the detailed description including preferred embodimentsof the present invention. The foregoing description including preferredembodiments of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible within the scope of the foregoing teachings.Additional variations of the present invention may be devised withoutdeparting from the inventive concepts as set forth in the followingclaims.

What is claimed is:
 1. A HEMT device comprising: a. a substrate; b. abuffer layer disposed above said substrate; c. a carrier supplying layerdisposed above said buffer layer; d. a gate element penetrating saidcarrier supplying layer; e. a drain element disposed on said carriersupplying layer; f. wherein the carrier supplying layer has anon-uniform thickness between said gate element and said drain element,the carrier supplying layer having a relatively greater thicknessadjacent the drain element and a relatively thinner thickness adjacentthe gate element.
 2. The HEMT device of claim 1 wherein the buffer layercontains GaN and wherein the carrier supplying layer contains AlGaN. 3.The HEMT device of claim 1 wherein the buffer layer and the carriersupplying layer comprise different group III-Nitride compounds.
 4. TheHEMT device of claim 1 wherein a two-dimensional electron gas conductionchannel is formed in said carrier supplying layer, the two-dimensionalelectron gas conduction channel having a non-uniform profile betweensaid gate and drain elements.
 5. The HEMT device of claim 1 wherein saidcarrier supplying layer smoothly varies in thickness between said gateelement and said drain element, the carrier supplying layer having itsrelatively greater thickness next to the drain element and itsrelatively thinner thickness next to the gate element.
 6. The HEMTdevice of claim 5 further including a source element disposed on saidcarrier supplying layer, said carrier supplying layer maintaining anessentially constant thickness between said gate element and said sourceelement.
 7. The HEMT device of claim 5 further including a sourcedisposed on said carrier supplying layer, said carrier supplying layermaintaining an essentially constant thickness between said gate elementand said source.
 8. A HEMT device having a non-uniform two-dimensionalelectron gas conduction channel formed in a carrier supplying layer ofthe HEMT device between a gate element of the HEMT device and a drainelement of the HEMT device, the HMET device also having a constantelectric field distribution between said gate element and said drainelement.
 9. The HEMT device of claim 8 further including a uniformtwo-dimensional electron gas conduction channel formed in said carriersupplying layer of the HEMT device between said gate element of the HEMTdevice and a source element of the HEMT device.
 10. A HEMT devicecomprising: a. a substrate; b. a buffer layer disposed above saidsubstrate; c. a carrier supplying layer disposed above said bufferlayer; d. a gate; e. a drain disposed on said carrier supplying layer;wherein a two dimensional electron gas (2DEG) is formed between the gateand the drain and wherein the carrier supply layer is configured toadapt the 2DEG such that a variation in electric field strength as afunction of a distance between the gate and the drain is substantiallyconstant.
 11. The HEMT device of claim 10 wherein the buffer layercontains GaN and wherein the carrier supplying layer contains AlGaN. 12.The HEMT device of claim 10 wherein the buffer layer and the carriersupplying layer comprise different group III-Nitride compounds.
 13. TheHEMT device of claim 10 wherein a two-dimensional electron gasconduction channel is formed below a heterointerface between the carriersupplying layer and buffer layer, the two-dimensional electron gasconduction channel having a non-uniform profile between said gate anddrain.
 14. The HEMT device of claim 10 wherein the gate penetrates saidcarrier supplying layer.
 15. The HEMT device of claim 10 wherein saidcarrier supplying layer smoothly varies in thickness between said gateelement and said drain element, the carrier supplying layer having itsrelatively greater thickness next to the drain element and itsrelatively thinner thickness next to the gate element.